Firebase. Storage() Is Not a Function in Javascript

I'm new to web development and firebase and a error is appearing to me. I've not been able to solve it so came here to seek for help.

The error displayed is firebase.storage() is not a function. It seems that the firestorage is not set correctly in my computer. However, I've followed the instructions download from the google website itself and it did not change a thing.

I've seen some other questions regarding this on Stack Overflow, but they use node or react which is not my case. Can someone give me a hint?

I know it is not explained in details because, as I said before, I'm not an expert. Please be compreensive and I'll develop better my question.

The code I've created is bellow. The first block is a subset of an .html file. The other one is the body of my .js file

  <!-- Firebase App is always required and must be first -->
      <script src=""></script>
      <!-- Add additional services that you want to use -->
      <script src=""></script>
      <script src=""></script>
      <script src=""></script>
      <script src=""></script>
      <script src=""></script>

      <!-- Inicializa o firebase -->
      <script>
          var config = {
              apiKey: "..",
              authDomain: "..",
              databaseURL: "..",
              projectId: "..",
              storageBucket: "..",
              messagingSenderId: "..",
              appId: ".."
          };
          firebase.initializeApp(config);
      </script>
      <!-- Nosso app -->
      <script src="./app.js"></script>

var fileInput = document.getElementById('file-input');
var stringInput = document.getElementById('string-input');

var ref = firebase.storage();

fileInput.onchange = function (event) {
}

stringInput.onchange = function (event) {

}